The invention relates to a method of manufacturing glass bodies, in which the starting material for the glass body, being monodispersed SiO.sub.2 particles (quartz-glass powder), is used to form an open-pore green body which is subjected to a purification process in which the impurities present in the green body react with a purifying gas which is heated to a temperature in the range from 600.degree. to 900.degree. C., after which the green body is sintered.
The invention further relates to the use of the glass bodies manufactured by the method in accordance with the invention.
The method mentioned in the opening paragraph is particularly suitable for the manufacture of preforms for optical waveguides of quartz glass.
Optical waveguides have a wide range of application; they are used, for example, in short-distance light-transmission arrangements or long-distance light-transmission arrangements as used in optical communication systems, and they are predominantly made of a type of glass which has a high silicon-dioxide content (which, if required, contains a dopant for adjusting the refractive index of the glass).
The types of glass which are suitable for the manufacture of optical waveguides can also advantageously be used for the manufacture of lamp envelopes for halogen lamps or gas-discharge lamps, because these types of glass, like the ones used for optical waveguides, must be substantially anhydrous and contain a high silicon-dioxide content.
For the manufacture of preforms for optical waveguides, for example, microdispersed colloidal SiO.sub.2 suspensions can be used which are formed into a green body. A method is known from, for example, the German patent application DE No. 3001792, in which through a sol-gel transformation a silicon alcoholate (or silicon alkoxide)/water/alcohol solution is hydrolized to form a gel in a vessel of a specific shape, after which the gel is slowly dried and sintered at a temperature below the melting temperature of the dry gel. By means of such a method, the desired shape can relatively easily be imparted to the green body, however, drying the gel so that a monolithic green body is obtained, which process must in any case be carried out so that no cracks are formed in the green body, presents considerable difficulties. The drying process must be carried out either extremely slowly, which is very time-consuming, or a large investment in equipment must be made (e.g. drying under hypercritical conditions in autoclaves). Purifying the dry green body in a heated gas atmosphere which reacts with the impurities present also presents difficulties due to the very large surface areas (typical size 1000 m.sup.2 /g) and, consequently, the very small spaces between the extremely fine particles. Moreover, upon sintering green bodies which are manufactured in this way, pin holes are often formed due to the adsorption of gas molecules at the surface and their subsequent occlusion upon sintering. A further disadvantage is that only green bodies can be formed which have a small relative green body density of .apprxeq.10% of the density of compact quartz glass.
The British patent GB No. 682580 describes, for example, a centrifuging method of manufacturing porous glass tubes for use as, for example, filters for laboratory purposes, in which method glass-powder suspensions are introduced into a binding agent which precludes settling of the solid particles, and to which wetting agents and agents which present foaming of the suspension are added, the assembly being introduced into a centrifuge on the inner wall of which a loose-textured structure of deposited solid particles is formed.
In a centrifuging process for forming green bodies for the manufacture of quartz-glass bodies, the wide particle-size spectrum of the commercially available microdispersed SiO.sub.2 particles is disadvantageous because of the very different settling rates of the individual particles and the consequent inhomogeneity of the green body obtained.
Quartz-glass powders as starting material for the manufacture of quartz-glass bodies, for example, optical waveguides, can, for example, also be processed in the form of extrusion material. The British patent GB No. 1010702 describes, for example, a method in which powdered, pure or substantially pure SiO.sub.2 with a liquid binding agent, and, if necessary, a further lubricant which facilitates the extrusion process is processed to form an extrusion material and then deformed in an extrusion process.
Upon processing microdispersed starting powders having particle diameters &lt;1 .mu.m, as for the manufacture of quartz-glass bodies (in particular also for preforms of optical waveguides), problems arise with the mixing or kneading, particularly when the starting materials contain a high content of microdispersed phase, because in this case a considerably greater amount of particules must be uniformly distributed, and a correspondingly large surface must be uniformly covered with the additives (binding agents and lubricants). For example, the typical mixing and kneading times for a starting material of microdispersed quartz-glass particles (10-100 nm diameter) containing 60% by weight of SiO.sub.2 ; 36% by weight of water and the remainder additives is approximately 1 to 3 hours.
Processing of powdered SiO.sub.2 particles has certain advantages over the sol-gel method of manufacturing the starting material for quartz-glass bodies. A disadvantage of the commercially available SiO.sub.2 powders is, however, that they are available only in highly dispersed form, i.e. in a wide range of particle sizes.
When monodispersed SiO.sub.2 powders are used, the abovedescribed difficulties can be reduced. The manufacture of monodispersed quartz-glass powders is known (J. Colloid Interface Sci. 26 (1968), pages 62 to 69). This known synthesis of SiO.sub.2 powders is based on the ammoniacal hydrolysis of alkoxysilanes dissolved in an alcoholic solution.
It has however been found that the spherical SiO.sub.2 particles obtained in this known manner are themselves porous. Upon processing such porous SiO.sub.2 particles the same difficulties present themselves as with, for example, SiO.sub.2 starting material which were manufactured by the sol-gel method: it is very difficult to purify green bodies formed from porous SiO.sub.2 particles in a heated gas atmosphere which reacts with the impurities present because of the very large specific surfaces of such green bodies (typical size 200-300 m.sup.2 /g). Moreover, upon sintering green bodies which are manufactured in this way, pin holes are often formed due to the absorption of gas molecules at the surface and their subsequent occlusion upon sintering.